It has been a practice in the past, in powering a two-axle, four-wheel drive earth-moving scraper, to provide engines close-coupled to the respective axles, using a larger engine in the forward position. This has been true of both push-loaded scrapers and elevator type scrapers. Further it has been conventional to employ articulated or wagon type steering at the front axle, with maximum steering angles of approximately 90 degrees to either right or left and with transverse oscillation of a front axle and cab through a range of about 20 degrees each way from level. Because of the concentration of power at the front axle, large front tires have been necessary. This has required the gooseneck of the draft frame to be arched high enough to clear the tires for any combination of steering angle and oscillation. Moreover, because of the large tires, it has been necessary to place the bowl of the scraper sufficiently to the rear of the front tires to insure clearance under all conditions. Because of the necessary clearance geometry and concentration of power in the tractor, prior tractors have been relatively large and heavy, having a height significantly greater than the height of the scraper section.
The use of a large engine in the tractor immediately adjacent to, and often somewhat ahead of, the operator's position makes his environment less than ideal. Noise and vibration as well as heat and fumes all tend to be at a high level, arising not only from the engine but from its ancillaries such as radiator and exhaust system. Forward driving visibility is reduced. The large front tires place the operator at a greater height above the ground which aggravates the severe rolling experienced by the operator as he oscillates transversely, along with the front axle, over rough terrain.
In addition the use of a large tractor section at the front of the vehicle tends to make the vehicle unstable, particularly at steering angles near the maximum where the rear wheels are the only major stabilizing means which must be relied upon to balance the front axle forces. Such instability is particularly great when the vehicle travels empty, and often at high speed, back to pick up a new load. Following prior practices there is considerable risk of overturning when traversing rough terrain.
Prior practices can, in addition, be shown to be economically disadvantageous. Constructing a tractor having high power capability with large tires and resulting large values of clearance geometry require all load-carrying elements to be of similar scale. The gooseneck and draft frame must not only be capable of withstanding a heavy load, but, because of the required increased span, such elements must be capable of accommodating loading applied at increased moment arms, which requires still further reinforcement. The resulting increase in weight necessary in the empty vehicle subtracts from the rated load capability within the gross weight capability of the vehicle, so that operating cost per delivered cubic yard is increased. In addition the dynamic stresses imposed upon both the operator and the vehicle tend to reduce operating speed and scraper productivity.
Closely related to the above is the fact that prior techniques result in poor "cubics", that is, the volume of earth which can be moved measured as a fraction of the volume occupied by the vehicle itself. This is due to the size and height of the tractor as compared to the size and height of the scraper bowl, which is not only a disadvantage during operation but also during shipment, storage and shop maintenance.
The use of two engines to accomplish a four-wheel drive has the following disadvantages: The reliability of the vehicle is reduced since two engines are susceptible to failure, and failure of either one of them puts the vehicle out of commission. Moreover, with two engines, two sets of ancillary equipment must be provided such as air cleaners, oil filters, exhaust systems, fans and radiators, and service checks and major repair must be performed for each. Relatively large spaces must be provided both on the tractor and trailer to accommodate two engines, their vibration mountings, enclosures and ancillaries.
It has been suggested in the past that the rear axle might be driven electrically. This has been found to be disadvantageous, however, because of low efficiency and since induction motors providing only a single speed approximately in low gear at the front drive have not offered good vehicle performance at higher speed. Efforts to improve performance by providing for a second motor speed has been at the expense of considerable investment in switchgear, with reduced reliability due to that switchgear. Even so the motor has been required to operate a good deal of the time under conditions of high slippage, resulting in overheating and possible damage. These problems are aggravated in climbing a grade due to the weight shift from the front axle to the rear of the vehicle.
Efforts have also been made to provide electrical propulsion in the driving of all four wheels, but this has been found to be disadvantageous since electrical propulsion systems have tended to be larger, heavier and more costly than mechanical drives. Moreover, electrical propulsion systems tend to be limited in the speed and torque ranges over which they can transmit full horsepower, and, because of the reduced efficiency inherent in an electric drive, total availability propulsion horsepower is reduced.
The above comments apply to scrapers in general. In the case of elevator type scrapers, power being provided from the front engine, difficulty has been experienced in coordinating elevator speed and performance with the propulsion speed and tractive effort, to keep the elevator from being overloaded and to avoid spinning and rapid wear of tires while maintaining reasonable efficiency.